Nickel-iron magnetic sheet stock



United States Patent Ofifice 3,297,434 Pa ent d Jen- 10, 7

3,297,434 NICKEL-IRON MAGNETIC SHEET STOCK Martin F. Littmann and Edwin S. Harris, Middletown,

Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Filed July 19, 1965, Ser. No. 473,175 g Claims. (Cl. 75124) This is a continuation-in-part of the copending application of the same inventors, Serial No. 316,118, filed October 14, 1963, now Patent No. 3,247,031, and entitled, "Method of Hot Rolling Nickel-Iron Magnetic Sheet Stock.

Nickel-iron alloys in which the named elements exist in about equal quantities have hitherto been made into relatively thin magnetic sheet stocks. These sheet stocks are characterized by relatively rectangular hysteresis loops and by a grain orientation responding to the designation (100) [001] by Millers Indices. This orientation is sometimes called cubic texture. The stocks find utility, inter alia, in the manufacture of cores for magnetic amplifiers and the like.

The fundamental object of the invention is the provision of nickel-iron magnetic sheet stocks having greatly improved magnetic qualities, and processes which can be depended upon to give a product having the improved characteristics.

This basic object and others which will be set forth specifically hereinafter are attained in that product and by the use of those procedures of which certain exemplary embodiments will now be described.

1. GENERAL DISCUSSION One of the specific objects of the invention may be stated as the consistent provision of a material of lower coercive force and a higher degree of rectangularity in thicknesses approximately in the range of A to 14 mils.

The products of this invention, measured in terms of magnetic properties under D.C. magnetization have:

(a) A magnetic induction exceeding 14.75 kilogausses at 1 oersted,

(b) A ratio of residual to peak induction not less than .96 at a peak magnetizing force of 1 oersted, and

(c) A coercive force of not more than about .09 oersted for material one to two mils thick, not more than about .15 oersted for materials 4 to 6 mils thick, and not more than about .25 oersted for thicker material up to 14 mils, measured at a peak magnetizing force of l oersted.

In terms of AC. magnetic properties measured by CCFR tests according to the AIEE Standard 432, the materials of this invention have:

(a) A magnetic induction exceeding 14.75 kilogausses at a peak magnetizing force of 1 oersted.

(b) A ratio of residual to peak induction not less than .96, and

(c) A value of H and AH as follows:

The materials of this invention are made by processes involving a final anneal not in excess of 2200' F. They have the [001] texture with a matrix or primary grain size not greater than 5.0 ASTM at 100x. The primary grains occupy by far the greater part of the surface area of the material. Less than 10% of the area occupied by secondary grains will be made up of grains exceeding 2 mm. in diameter at a magnification of IX.

So far as is known the combination of qualities set forth above has not hitherto been attained in magnetic sheet stocks containing substantially equal parts of nickel and iron.

Researchers have shown that the magnetic induction (B is affected by the gross compositions of the product and by the grain orientation. The ratio of residual to peak induction (B /E which is indicative of the squareness of the hysteresis loop is dependent basically on the grain orientation of the product. The coercive force (H which determines the narrowness of the hysteresis loop, is dependent upon a number of factors including the annealing temperature and the impurities contained in the material. It may be stated that a hysteresis loop which is narrow in the horizontal direction is desired. AH is the function of the slope of the magnetization curve and is dependent upon grain orientation, grain size and any residual stresses there may be in the product. A relatively small grain size is desirable.

It will be seen from the above that the attainment of the objects of the invention is dependent upon the 00- action of a number of factors not only of composition but also of treatment in the production of the material.

2. COMPGSITION In the practice of the invention an alloy steel is made by any suitable refining or melting technique and having the following initial composition:

Niabout 47.5% to about 48.5%, or roughly 47% to 49%.

Cabout .005 to about 019% but in any event less than about 035%.

v Mnfrom about 30% to about 1.0%, and preferably in the range of about 35% to about .70%.

Pnot greater than about .005

Sfrom about .0030% to about .010%, but preferably in the range of about .0040% to about .0075-%.

Sifrom about 30% to about 50%, but preferably in the range of about 35% to about .45

Al--from about 010% to about 035%, but preferably from about .015 to about 025%.

Fethe balance of the alloy excepting for residual impurities in trace amounts. The quantity of iron should be approximately equal to the quantity of nickel in the alloy. Residuals should be kept as low in quantity as practicable.

The sulfur has an effect on controlling the grain growth in the product, and tends to inhibit secondary recrystallization. Enough sulfur should be present initially to prevent premature secondary growth which would increase AH and lower the B /B ratio. By the termination of the processing, however, enough sulfur must be removed to obtain low H and H values.

. The control of the total alloying additions should be strict if high saturation (high B values) is to be maintained.

It has been found that the phosphorus present has an important effect in controlling the ultimate orientation of the grains in the product. Excessive phosphorus impairs the orientation even of the primary grains, and this would result in low 13 and B /B values and in increase in AH.

Aluminum has an effect upon grain growth in the product and helps to prevent secondary growth. The

best B /B values are attained with an aluminum con:

tent of about 020%, which is regarded as optimum.

The silicon permits better control of oxygen during the melting procedure. In the amounts indicated it tends to reduce B B /B and H Too much silicon will promote premature secondary grain growth and tends to reduce the B /B value.

Nickel when used as described, and especially in the preferred range, provides the best magnetic saturation.

The carbon should be restricted as indicated.

The melt analysis as set forth above is important because it is necessary to the practice of the invention that the alloy contain the stated elements in the stated proportions during the processing and particularly during the hot rolling and hot coiling treatments hereinafter described. The final product in sheet or strip form will be found upon analysis to contain the same elements in substantially the same proportions excepting that a change will occur in the content of sulfur and a minor change may occur in the content of carbon. The final sulfur, as will later be shown, should not be above about .0020%. Some carbon may be lost during processing; but the procedure herein taught does not include a decarburizing step as known in the art, and the dry atmosphere of the final anneal, and an intermediate anneal if practiced, cannot under the conditions described effect any large reduction in carbon content. While somewhat lower carbons are frequently achieved, the value of .035 previously given can .still be regarded as the maximum both for theinitial material and for the final product. A substantially lower carbon content is usually the result of using a low carbon melt.

3. HOT ROLLING A melt of the analysis set forth above is teemed into ingot molds, preferably molds having cross sectional dimensions of 16" x 29", but without limitation since other sizes may be employed. The molds will be stripped from the ingots, and the ingots are placed in soaking pits and brought to a temperature of about 2200 F. to 2300 F., preferably about 2250" F.

When the ingots are uniformly at a temperature within the indicated range they are preferably hot rolled in known ways to slabs which are 3" to 6" in thicknes. The slabs are cooled and conditioned in known ways to remove scale and other defects. Alternatively, the slabs may be continuously cast.

Instead of slabbing the ingots as indicated, it is possible to roll the ingots directly to an intermediate hot rolled gauge as later specified; but if this is done precautions should be taken to see that the temperature of the metal will not exceed the ranges set forth in the next following paragraph when the metal is at comparable thick nesses.

When the preferred procedure is followed, the conditioned slabs are reheated to a temperature within the range of about 1850 F. to about 2050 F. but preferably in a narrower range of about 1900 F. to about 2000 F. The slabs are then roughed down to a thickness of about .450" and then finished in finishing stands to a thickness in the range of about .125" to about .180" but preferably from about .140 to about .155". The finishing temperature should lie within the range of about 1300 F. to about 1500 F. but preferably in the narrower range of about 1360 F. to about 1420 F. The hot bands formed in this way will be coiled at a temperature not exceeding about 1100 P. All temperatures are based on measurements taken with radiation pyrometers calibrated under black body conditions.

The temperature requirements are set forth are applicable whether or not the routing includes an intermediate slabbing treatment, and the temperature requirements must be adhered to strictly. The effect of finishing and coiling the hot bands at too high a temperature will resuit in a grain size in the hot band which is too large to permit the attaining of the objects of this invention. If the hot band is finished at too low a temperature there will be apparent in it an effect of too much cold reduction, which will interfere with the desired grain orientation.

4. COLD ROLLING After pickling to remove scale, the hot rolled band will be cold reduced to a final thickness of from A to 14 mils. If the cold rolling can be done to a chosen final gauge without intermediate annealing, this is desirable. For example, it is possible to cold roll the pickled hot band to a thickness of about .025" on an ordinary 4-high cold mill; and the remainder of the cold rolling will then be accomplished on a mill of a type designed to do extremely fiat rolling with small working rolls. An example of such a mill, but without limitation, is the well known Sendzimir mill having working rolls not over about 1% in diameter. Reductions may be made to a final gauge of about .014" to about .002" without intermediate annealing. It is an advantage of the process that no anneal following t-he hot rolling but preceding the cold rolling is required.

When a final thickness of 1 mil or less is desired, an intermediate anneal will usually be practiced. The strip thickness at this anneal should be about 50 to times the final thickness. The anneal is conducted at about 1600 F. in a reducing atmosphere for a time of about 1 to 2 minutes.

When the material has been reduced to gauge it may then be slit into strips of the desired width, and cores may be fabricated from the strips by coating them with an annealing separator such as M O, and winding them to a toroidal core formation.

5. FINAL ANNEAL The cores formed as above described are subjected to a final anneal in a temperature range of about 1800 F. to about 2200 F. for about one to four hours at temperature in dry hydrogen, i.e., hydrogen having a dew point not over about 10 F.

This anneal serves to recrystallize the material and to cause annealing twins to be adsorbed so that a uniformly fine grain structure results with the cubic or (100) [001] texture. A sudden grain growth or secondary recrystallization will occur if the annealing temperature is higher than appropriate for the amount of sulfur present or the degree of cold reduction. Secondary grains over 2 mm. in diameter must be avoided in amounts exceeding 5% to 10% of the area of the material to retain a satisfactory level of AH.

There are optimum amounts of sulfur which vary with the temperature of the final heat treatment from about .003% for a heat treatment at about 1800 F, to about .006% for a heat treatment within the range of about 2150 F. to about 2200 F. At the same time the final heat treatment in dry hydrogen results in the removal of sulfur which permits the attainment of a high degree of the cubic texture. Samples of the steel tested for sulfur content after the described final anneal were found to contain .0010% to .0020% sulfur.

Example A 27,000 pound heat of nickel-iron alloy was melted in an arc furnace and was cast into 16" x 29" ingots weighing 9,000 pounds each and having a composition as follows:

Percent s Si .40

Fe Balance The ingots were heated to 2275 F and were rolled into slabs 5" thick, which were then cooled.

The slabs were ground and reheated to 1920 F. to 1945 F. and were roughed into bars .45 thick. The bars entered the finishing mills at 1550 F. to 1580 F. and were rolled to .145" at a finishing temperature of 1385 F. to 1400 F. The hot rolled intermediate gauge product was cooled with water on a run-out table and then was coiled at a temperature of 980 F. to 1030 F.

After pickling, the coils of the intermediate gauge hot rolled product were cold rolled to .025 using 16 diameter working rolls. Thereafter the material was carried down to a thickness of .002" using a mill having 1.2" diameter working rolls.

The sheet gauge material was slit to form strips 4 wide, which strips were then coated with dry magnesia using a light film of lubricating oil to cause the magnesia to adhere to the strips. The strips were wound into toroidal cores having an inside diameter of 1" and an outside diameter of 1 A". The cores were annealed in a mufile using dry hydrogen as the annealing atmosphere, i.e., hydrogen having an exit dew point below -60 F. Three batches of the cores were annealed at different temperatures as shown in the table below; but the time at temperature in each instance was two hours.

The results of CCFR tests were as follows:

Anneal Temperature Bm, K a BrlBm H Oer. AH Oer.

2,100 F 15. 45 979 185 0164 2,l50 F 15. 37 978 181 0153 2,200 F 15. 42 978 171 0158 Ni 47% to 49%.

C Less than .035%.

Mn .30% to 1.0%.

P Less than .005%.

S From .0030% to .010%. Si From .30% to 50%.

Al From .010% to .035%.

Balance, excepting for residual Fe impurities in trace amounts.

2. The nickel-iron alloy claimed in claim 1 in the form of sheet stock having a final gauge of about A to about 14 mils, and having a composition in finished form containing the recited ingredients in substantially the recited proportions excepting for a reduction in the sulfur content such that the sheet stock contains not more than about .0020% sulfur.

3. The sheet stock of claim 2 having a magnetic induction exceeding 14.75 kilogausses at a peak magnetizing force of 1 oersted, a ratio of residual to peak induction not less than about .96, an H; value not greater than about .20 to about .32 oersted varying with the gauge 6 within the range given, and a value of AH not greater than about .022 to about .060 oersted, again varying with the gauge.

4. The sheet stock claimed in claim 3 having a grain orientation of the [001] type by Millers Indices, the greater part of the surface area of the sheet stock being made up of primary grains having a size not greater than about 5.0 ASTM at 100x, the remaining surface area being made up of secondary grains, less than 10% of the last mentioned area being occupied by secondary grains exceeding 2 mm. in diameter at 1X.

5. A nickel-iron alloy for magnetic purposes having an initial composition consisting of the following ingredients in substantially the proportions stated:

Ni 47.5% to 48.5%.

C .005% to 019%.

Mn .35% to .70%.

P Less than .005%.

S .0040% to .0075%.

Si 35% to .45%.

Al .015% to .025%.

Fe Balance, excepting for residual impurities in trace amounts.

6. The nickel-iron alloy claimed in claim 5 in the form of sheet stock having a final gauge of about A to about 14 mils, and having a composition in finished form containing the recited ingredients in substantially the recited proportions excepting for a reduction in the sulfur content such that the sheet stock contains not more than about .0020% sulfur.

7. The sheet stock of claim 6 having a magnetic induction exceeding 14.75 kilogausses at a peak magnetizing force of 1 oersted, a ratio of residual to peak induction not less than about .96, an H value not greater than .20 to about .32 oersted varying with the gauge within the range given, and a value of AH not greater than about .022 to about .060 oersted, again varying with the gauge.

8. The sheet stock claimed in claim 7 having a grain orientation of the (100) [001] type by Millers Indices, the greater part of the surface area of the sheet stock being made up of primary grains having a size not greater than about 5.0 ASTM at 100x, the remaining surface area being made up of secondary grains, less than 10% of the last mentioned area being occupied by secondary grains exceeding 2 mm. in diameter at 1X.

9. A nickel-iron alloy for magnetic purposes in sheet form and consisting of the following ingredients in substantially the proportions stated:

Ni 47% to 49%.

C Less than 035%.

Mn .30% to 1.0%.

P Less than .005%.

S Not gre ater than about Si .002O%.

Al From .30% to 50%.

Fe From 010% to 035%.

Balance excepting for residual impurities in trace amounts.

10. A nickel-iron alloy for magnetic purposes having a composition consisting of the following ingredients in substantially the proportions stated:

impurities in trace amounts.

(References on following page) References Cited by the Examiner 2,558,104 6/1951 Scharschu 148100 UNITED STATES PATENTS 2,569,468 10/ 1951 Gaugler 148120 X 12 1930 Brandsma 14 31 55 X 2,891,883 6/1959 Howe 14831.55 X

6/1932 Ruder 14831.55 X 5 6/ 1932 R d 148 3 1 5 5 X DAVID L. RECK, Primary Examiner,

7/1932 Seastone 148-31.55 X 2/1939 Holst 148 210 P. WEINSTEIN, Asslstant Examznen, 

1. A NICKEL-IRON ALLOY FOR MAGNETIC PURPOSES HAVING AN INITIAL COMPOSITION CONSISTING OF THE FOLLOWING INGREDIENTS IN SUBSTANTIALLY THE PROPORTIONS STATED: 